diff --git a/private_gpt/components/prompts/prompt_builder.py b/private_gpt/components/prompts/prompt_builder.py
index f7665889..938db329 100644
--- a/private_gpt/components/prompts/prompt_builder.py
+++ b/private_gpt/components/prompts/prompt_builder.py
@@ -117,7 +117,12 @@ class PromptBuilderService:
             assistant_name=assistant_name or settings().chat.assistant_name,
             assistant_description=assistant_description
             or settings().chat.assistant_description,
-            current_date=current_date.astimezone().isoformat(),
+            # Use date-level granularity: a sub-second timestamp here changes on
+            # every request and sits at the front of the system prompt (before the
+            # guidelines and retrieved context), which defeats LLM prompt-prefix
+            # caching (e.g. OpenAI automatic prefix caching, local KV-cache reuse).
+            # The model only needs the calendar date for relative-date reasoning.
+            current_date=current_date.astimezone().date().isoformat(),
         )
 
     def create_chat_context_for_system_prompt(